Protective means for transmitters



' Nov. 16, 1943. p ANDREWS 2,334,530

PROTECTIVE MEANS FOR TRANSMITTERS Filed May 9, 1942 pagl. H

70 SOURCE OF 056/LL47/0/VS HES/$77719 HA V/A/G RE 8/8 7' 0/? HA VII/6 Fi 4, NEG/177V! TEMPERATURE Pi 6.

Inventor: aul DAndrewS,

by M MMA H i sAttorney.

Patented Nov. 16, 1943 PROTECTIVE MEANS FOR TBANSMITTEBS Paul D. Andrews, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application May'9, 1942, Serial No. 442,353

4 Claims.

My invention relates to means for protecting electron discharge devices employed in radio transmitters from damage tending to occur, for example, under certain conditions of undesirable output circuit change or detuning which may resuit from grounds, open circuits or other faults in the transmitter antenna or other load circuit of the system.

The invention is particularly applicable to means for the protection of discharge devices of certain radio transmitters or the like in the design and construction of which rigid minimum space and weight requirements are required to be met.

In Patent 1,972,698 to Paul D. Andrews issued September 4, 1934, and assigned to the same assignee as the present application, a protecting means for electron discharge devices of radio transmitters against damage from the above mentioned or like causes is described and claimed which includes in the keying circuit of the transmitter a time delay relay arranged to deenergize the keying relay circuit in case the key is held in closed position for more than some predetermined safe time period.

My present invention is an improvement upon the protective system disclosed in the above mentioned patent and its object is to provide a novel,

' simple and efiicient system of this character in which the size and weight of the protective apparatus are substantially reduced.

In accordance with the present invention this object is attained by providing for the purpose of deenergizing the keying relay circuit under abnormal keying conditions, a heatresponsive means so arranged that the conductivity thereof,

increases with the heat supplied thereto. The heat-responsive means may be preferably either a suitable electron discharge device so arranged that its cathode emission is small during normal keying and large during abnormal keying, or the heat-responsive means may be a suitable resistance element having a large negative temperature coeilicient and so arranged that its resistance value is large during normal keying and sentation of a radio transmitting system in which my invention has been embodied and employing an electron discharge device as a protective means; Fig. 2 is a modification similar to the embodiment of Fig. 1 and employing an auxillary relay, and Figs. 3 to 8 represent modifica- I tions employing as a protective means a resistance element having a negative temperature coefilcient.

Referring to Fig. 1 the radio transmitting sys' tem represented therein may comprise an electron discharge transmitter device or amplifier In of any usual or preferred form, upon the control electrode II of which oscillations are impressed from a suitable source (not shown) and the output circuit of which may be coupled through a transformer l2 to a load circuit which includes an antenna I I. The input circuit of the discharge device I ll may include a usual grid condenser l4 and a leak resistance I! in series with a high frequency choke l6.

Various means may be employed for keying the transmitter l0. As shown by way of example in Fig. l the transmitter may be maintained normally in non-conductive condition by impressing upon its control electrode II a bias potential which is maintained normally negative by the voltage drop through resistor I 1 connected in.

series with a second resistor [8 across the source (not shown) of anode potential for the transmitter. To cause the transmitter to become conducting in accordance with desired signals thereby to impress the signals upon the antenna l3, the resistor i1 is short-circuited at suitable periods by a keying relay or switch H in accordance with the operation of a key or other suitable keying device III. The relay l9 includes an energizing winding 2| adapted to close the normally open contacts I! when supplied with energizing current from a source 21 such as a battery or rectifier, upon closure of the key 20.

For a purpose which wilbbeexplained herein-, after the keying circuit 24 including the key 20, relay 2 l, and current source 2!, further includes a heat-responsive element constituted by an electron discharge device 25 connected in shunt with the winding 2|. The cathode II of device 25, or the cathode heater (not shown), if the cathode is of the indirectly heated type, is connected in series in the keying circuit. A separate source (not shown) of anode potential may be provided if desired for the discharge device 25. A limiting resistor 21 may also be connected in the shunt circuit including cathode 28, source 23, and key 20. In order to compensate for differences in required current between keying relay II and cathode 2B or the control discharge device 25 or for adjusting the operating characteristic of the latter device, an adjustable resistor 23 may be provided in shunt with the cathode 26 or cathode heater.

In normal operation of the system in transmitting signals from antenna II, the short pulses of energizing current supplied to relay winding 2|, and corresponding to the intermittent actuation of key in forming the signals, have only slight heating effect and thereby cause only slight electron emission from the cathode 23 of electron discharge device 25. Therefore, the latter device is normally in non-conductive condition and presents such a high impedance in shunt with the relay winding II that operation of the relay i9 is not appreciably affected by the presence of the device 25 in the keying circuit.

Let it be assumed, however, that a disarrangement or faulty condition of the antenna load circuit l3 occurs such as a short circuit, as illustrated, for example, at a point of the load circuit designated by the numeral 29. Under such conditions dangerous heating of the anode of the amplifier 10 ensues if the key is in closed position thereby holding the keying relay I! also in closed position. The amplifier may be destroyed or seriously damaged if the key 20 is maintained in its closed position longer than a relatively short period.

In accordance with my present invention such damaging effects are avoided, in case of a fault occurring in the antenna load circuit and with the key 20 in closed position, by the heat-respo sive impedance means represented in Fig. 1 by the electron discharge device 25, since the continuous closure of the key 20 for this relatively short period, of the order of five seconds for example, causes sumcient heating of cathode 26 to increase the electron emission thereof to such an extent that the latter discharge'device 2! then presents a relatively low impedance across the energizing winding 2| of keying relay 1!, thereby causing the latter winding to become deenergized and the contacts 22 to open and remain open.

With keying relay l9 thus maintained in open position the normal negative bias potential constituted by the voltage drop in resistor I1 is impressed upon control electrode ll of transmitter Ill and the latter returns to its normal non-conductive condition, thereby being protected from the damaging effects of the faults in the antenna circuit.

In Fig, 2 the arrangement and operation of the system illustrated therein are essentially as in the system of Fig. 1 except that the variable shunt impedance provided by the electron discharge device 25 is arranged in Fig. 2 to control the energizing of the actuating winding of an auxiliary relay 3!, the contacts 32 of which are connected in series in the keying circuit to control the energizing of the actuating winding 21 of the main keying relay iii. A resistor 33 in shunt with the winding 30 may be provided for adjustment of the operating characteristics of the auxiliary relay circuit. The auxiliary relay 3| may be a standard telephone relay and hence small, light, and of low cost.

Referring to Fig. 3, the system illustrated therein differs from that of Fig. 1 only in that, whereas in Fig. 1 the heat-responsive impedance shunting the keying relay I9 is an electron discharge device, in Fig. 3 the heat-responsive impedance element is constituted by a resistance element 34 having a high negative temperature coefficient.

In normal operation of the system of Fig. 3, the short pulses of current through the element ll corresponding to the short periods of closure of key 20 have little or no heating effect and the resistance element therefore remains in its normal high resistance condition, the operation of keying relay l9 thus being unaffected by the presence of the shunting resistance element 3| in the keying circuit. In case key 20 is maintained continuously in closed position, however, the heating effect upon resistance element 3| after a predetermined short period is suiilcient to cause its resistance value to drop to a relative- 1y low point, whereby the impedance presented by the element 34 shunting the winding 2| is so decreased that the relay contacts 22 open, thereby protecting the system from damage as explained in connection with Fig. 1.

Suitable materials for the negative temperature coefllcient resistance element 34, such as uranium oxide, 9. mixture of nickel oxide and manganese oxide, and silver sulphide, are described in Bell Laboratories Record of December, 1940, pages 106-111, in an article entitled Thermistors, their characteristics and uses.

In Fig. 4 the arrangement and operation of the system shown therein are the same as in the system of Fig. 3 except that in Fig. 4 the negative temperature coeillcient resistance element 3 is adapted to be heated partially or substantially entirely by a heater element 35 so arranged as to be heated, as from source 23, in accordance with the closure periods of the keying device 20.

In Fig. 5 the system is essentially the same as that illustrated in Fig. 2 except that in Fig, 5 the auxiliary relay 3i is energized through the negative temperature resistance element 34. The system of Fig. 6 differs from that of Fig. 5 in that in Fig. 6 an indirect heater means is provided for the resistance element 34 as in Fig. 4.

My invention has been described herein in particular embodiments for purposes of illustration.

. It is to be understood, however, that the invention is susceptible of various changes and modiilcations and that by the appended claims I intend to cover any such modifications as fall within the true spirit and scope or my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In combination, a transmitter, a keying circuit therefor including a relay and a key to control energization thereof, and a heat-responsive impedance means characterized by increased electrical conductivity with increased heat supplied thereto arranged to be heated by current flowing in the keying circuit when the key is in operative position and operatively associated with said keying circuit for deenergizing said relay when said key remains in operative position longer than a predetermined time period.

2. In combination, a transmitter, a keying circuit therefor including a switch and keying means to control energization thereof, and an electron discharge device having a cathode controlled by said keying means, said device being operatively associated with said circuit for deenergizing said switch when said keying means remains in operative position longer than a predetermined time period, said circuit and said discharge device being so arranged that the cathode emission of said device is substantially increased Within said time period.

3. In combination, a transmitter, a keying cir cuit therefor including a switch and a keying device to control energization thereof, and an electron discharge device for deenergizing said switch when said keying device remains in operative position longer than a predetermined time period, said discharge device being connected in shunt with said switch and including a cathode energized by said keying device, said circuit and said discharge device being so arranged that the electron emission from said cathode is small durin norm operation of said keying device and is increased substantially within said time period.

4. In combination, a transmitter, a keying circuit therefor including a switch and a keying device to control energization thereof, and a heatresponsive resistance element having a negative temperature coeii'icient and connected in shunt with said switch so as to be heated by current due to the voltage across the switch for deenergizing said switch when said keying device remains in operative position longer than a prede- PAUL n. mpnnws. 

